This invention relates to a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet good in corrosion resistance and surface appearance and a method of producing the same.
It is known that steel sheet immersed in a hot-dip plating bath of zinc containing an appropriate amount of Al and Mg to plate the steel sheet with this alloy exhibits excellent corrosion resistance. Because of this, various avenues of research and development have been pursued regarding this type of Znxe2x80x94Alxe2x80x94Mg-system. Up to now, however, no case of a plated steel sheet of this system having achieved commercial success as an industrial product has been seen.
The specification of U.S. Pat. No. 3,505,043, for example, teaches a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet with excellent corrosion resistance using a hot-dip plating bath composed of Al: 3-17 wt. %, Mg: 1-5 wt. % and the remainder of Zn. This was followed by proposals set out in, for example, JPB-64-8702, JPB-64-11112 and JPA-8-60324 for improving corrosion resistance and productivity by incorporating various addition elements in the basic bath composition of this type, regulating the production conditions, and the like.
In industrial production of such hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet, while it is of course necessary for the obtained hot-dip plated steel sheet to have excellent corrosion resistance, it is also required to be able to produce a steel strip product good in corrosion resistance and surface appearance with good productivity. Specifically, it is necessary to be able to stably produce hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet with good corrosion resistance and surface appearance by continuously passing a steel strip through an ordinary continuous hot-dip plating machine commonly used to produce hot-dip galvanized steel sheet, hot-dip aluminum plated sheet and the like. In this specification, the term xe2x80x9chot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheetxe2x80x9d is for convenience used also for a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel strip produced by passing a steel strip through a continuous hot-dip plating machine. In other words, xe2x80x9cplated sheetxe2x80x9d and xe2x80x9cplated stripxe2x80x9d are defined as representing the same thing.
In the equilibrium phase diagram for Znxe2x80x94Alxe2x80x94Mg, the ternary eutectic point at which the melting point is lowest (melting point=343xc2x0 C.) is found in the vicinity of Al of about 4 wt. % and Mg in the vicinity of about 3 wt. %. In the production of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet based on a Znxe2x80x94Alxe2x80x94Mg ternary alloy, therefore, it would appear at a glance to be advantageous to make the composition close to this ternary eutectic point.
When a bath composition in the vicinity of this ternary eutectic point is adopted, however, a phenomenon arises of local crystallization of a Zn11Mg2-system phase in the metal structure of the plating, actually of an Al/Zn/Zn11Mg2 ternary eutectic crystal matrix per se or in this matrix of a Zn11Mg2-system phase including a primary crystal Al phase or a primary crystal Al phase an Zn single phase. This locally crystallized Zn11Mg2-system phase discolors more easily than the other phase (Zn2Mg-system phase). During standing, this portion assumes a highly conspicuous color tone and markedly degrades the surface appearance. The value of the plated steel sheet as a product is therefore manifestly degraded.
Through their experience, moreover, the inventors learned that when this Zn11Mg2-system phase locally crystallizes there arises a phenomenon of this crystallized portion being preferentially corroded.
An object of the invention is therefore to overcome this problem and to provide a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet good in corrosion resistance and surface appearance.
The inventors further learned that when the ordinary hot-dip plating operation of continuously immersing/extracting a steel strip in/from a bath is applied to a plating bath of this system, a stripe pattern of lines running in the widthwise direction of the sheet occurs. During production of Zn-base plated steel sheet containing no Mg, no such line-like stripe pattern occurs under normal conditions even if Al should be added to the bath, nor have cases of its occurrence been noted in hot-dip Al plated steel sheet. The inventors discovered that the Mg in the bath is involved in the cause, specifically that the stripe pattern of lines occurring at intervals in the widthwise direction of the steel sheet is peculiar to hot-dip galvanized steel sheet containing Mg.
The inventors believe the reason for this to be that a Mg-containing oxide film forms on the surface of the molten plating layer adhering to the steel strip immediately after extraction from the bath and that owing to this formation the surface tension and viscosity of the plating layer surface portion are of a special nature not found in hot-dip galvanized steel sheet, hot-dip Al plated steel sheet and the like. Overcoming the problem of this special nature is indispensable for industrial production of such plated steel.
One object of the invention is therefore to provide such steel sheet having a good appearance without such a pattern.
This invention provides a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet good in corrosion resistance and surface appearance that is a hot-dip Zn-base plated steel sheet obtained by forming on a surface of a steel sheet a hot-dip Znxe2x80x94Alxe2x80x94Mg plating layer composed of Al: 4.0-10 wt. %, Mg: 1.0-4.0 wt. % and the balance of Zn and unavoidable impurities, the plating layer having a metallic structure including a primary crystal Al phase or a primary crystal Al phase and a Zn single phase in a matrix of Al/Zn/Zn2Mg ternary eutectic structure.
In the metallic structure of the plating layer, preferably the total amount of the primary crystal Al phase and the Al/Zn/Zn2Mg ternary eutectic structure is not less than 80 vol. % and the Zn single phase is not greater than 15 vol. % (including 0 vol. %).
The hot-dip plated steel sheet having the plating layer of this metallic structure can be produced by, in the course of producing a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet using a hot-dip plating bath composed of Al: 4.0-10 wt. %, Mg: 1.0-4.0 wt. % and the balance of Zn and unavoidable impurities, controlling the bath temperature of the plating bath to not lower than the melting point and not higher than 450xc2x0 C. and the cooling rate up to completion of plating layer solidification to not less than 10xc2x0 C./s or controlling the bath temperature of the plating bath to not lower than 470xc2x0 C. and the post-plating cooling rate up to completion of plating layer solidification to not less than 0.5xc2x0 C./s.
The invention further provides a hot-dip Znxe2x80x94Alxe2x80x94Mg-system plated steel sheet good in corrosion resistance and surface appearance that is a hot-dip Zn-base plated steel sheet obtained by forming on a surface of a steel sheet a plating layer composed of Al: 4.0-10 wt. %, Mg: 1.0-4.0 wt. %, Ti: 0.002-0.1 wt. %, B: 0.001-0.045 wt. % and the balance of Zn and unavoidable impurities, the plating layer having a metallic structure including a primary crystal Al phase or a primary crystal Al phase and a Zn single phase in a matrix of Al/Zn/Zn2Mg ternary eutectic structure. In the metallic structure of this Ti/B-added plating layer, preferably the total amount of the primary crystal Al phase and the Al/Zn/Zn2Mg ternary eutectic structure is not less than 80 vol. % and the Zn single phase is not greater than 15 vol. % (including 0 vol. %).
In the case of this Ti/B-added hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet, a hot-dip plated steel sheet having a metallic structure including a primary crystal Al phase or a primary crystal Al phase and a Zn single phase in a matrix of Al/Zn/Zn2Mg ternary eutectic structure can be produced by using a hot-dip plating bath composed of Al: 4.0-10 wt. %, Mg: 1.0-4.0 wt. %, Ti: 0.002-0.1 wt. %, B: 0.001-0.045 wt. % and the balance of Zn and unavoidable impurities and controlling the bath temperature of the plating bath to not lower than the melting point and lower than 410xc2x0 C. and the post-plating cooling rate to not less than 7xc2x0 C./s or controlling the bath temperature of the plating bath to not lower than 410xc2x0 C. and the post-plating cooling rate to not less than 0.5xc2x0 C./s.
According to the invention, in order to control the stripe pattern of lines running in the widthwise direction of the sheet that readily arises in a Znxe2x80x94Alxe2x80x94Mg plated steel sheet of this type, it was found advantageous to subject the Mg-containing oxide film that forms on the surface layer of the molten plating layer adhering to the surface of the steel strip continuously extracted from the bath to morphology control until the plating layer has solidified, more explicitly, to regulate the oxygen concentration of the wiping gas to not greater than 3 vol. % or to provide a sealed box to isolate the steel sheet extracted from the bath from the atmosphere and make the oxygen concentration in the sealed box not greater than 8 vol. %.
Further, according to the invention, it was found that occurrence of the stripe pattern of lines in the widthwise direction of the sheet can be controlled by adding to the plating bath an appropriate amount of Be, specifically, 0.001-0.05% of Be. The invention therefore also provides a hot-dip Zn-base plated steel sheet with no stripe pattern produced using a hot-dip plating bath obtained by adding Be: 0.001-0.05 wt. % to a hot-dip Znxe2x80x94Alxe2x80x94Mg-system plating bath composed of Al: 4.0-10 wt. % and Mg: 1.0-4.0 wt. %, and, as required, Ti: 0.002-0.1 wt. % and B: 0.001-0.045 wt. %, and the balance of Zn and unavoidable impurities.